Staging: ipack: use idr interface for numbering buses

Use idr interface to give the bus number. That way, we remove the
limitation of 64 buses.

The mutex is removed because the ida interface uses spinlocks inside, so it is
not needed an extra lock.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>